The present invention relates to electromagnetic radiation sensor systems. In particular, the present invention relates to improved electrical interconnections for use in hybrid infrared focal planes.
Previous designs of cryogenically cooled infrared focal planes can be characterized by a limited number of photodetector elements, an absence of preamplifiers or other signal processing elements, and a large complement of signal leads. Preamplification and other signal processing is carried out external to the Dewar. The complexity of the focal planes has been limited by the number of signal leads which can be routed from inside the Dewar, through vacuum seals, to the associated electronics outside of the Dewar.
Recent developments in semiconductor technology have led to the feasibility of more complex infrared system focal planes, with more sensitivity, higher resolution, higher data rates and greater reliability. These developments include advances in microelectronics, especially in charge transfer devices, MOS technology, and large scale integration.
The advent of charge transfer devices and their supporting technology introduces several major benefits. First, it is now possible to consider carrying out a variety of signal processing tasks in situ on or near the focal plane. Second, the multiplexing capability of charge transfer devices permits a dramatic reduction in the number of leads leaving the focal plane.
The intergration of infrared detectors and integrated circuit signal processors on the focal plane, therefore, reduces the number of electrical feedthroughs and decreases the system complexity. Higher system performance becomes possible, along with considerable reduction in overall cost. An example of a proposed infrared detector/integrated circuit focal plane is shown in U.S. Pat. No. 3,883,437 by K. Nummedal et al.
The development of hybrid focal planes containing both infrared detectors and associated signal processing, however, presents several technical challenges. First, the typical infrared detector materials are mercury cadmium telluride, lead tin telluride, lead selenide telluride, and indium antimonide. The signal processing portions of the hybrid focal plane, however, will typically be silicon because the silicon technology is far more advanced than those of the typical infrared detector materials. In addition, the larger bandgap of silicon provides certain advantages over the narrower bandgaps of the typical infrared detector materials. The design of a hybrid focal plane, therefore, must accommodate infrared detectors and signal processors which are formed from different semiconductor materials.
Second, it is highly desirable to provide very high packing density of photodetectors on the focal plane. This complicates the interconnections of the photodetectors to the associated signal processing circuitry.
Third, since the photodetectors and the signal processing circuitry will be processed at different times, it is important that the processing of the infrared detectors does not adversely affect previously formed solid state signal processing circuitry. or vice versa.
From both a reliability and an ease of fabrication standpoint, any electrical interconnection used with a hybrid mosaic IR/CCD focal plane should use some form of thin film metalization. The soldering or welding of individual wires from contacts on the integrated circuit to individual detectors is not suitable for a hybrid mosaic IR/CCD focal plane.
One form of interconnect which has been suggested for use in hybrid mosaic IR/CCD focal planes is shown in FIG. 1 of the two previously mentioned co-pending patent applications. This interconnect involves the formation of a totally evaporated lead from a contact pad over a beveled side of a detector, and onto the top of the detector. A detailed description of the fabrication techniques used in producing totally evaporated leads of this type is also found in co-pending U.S. Patent Applications by R. V. Lorenze, Jr. and M. F. Young, Ser. Nos. 720,917 and 720,918, entitled "Method of Preparing Photodetector Array Elements" and "Photodetector Array Delineation Method", filed Sept. 9, 1976 and assigned to the same assignee as the present application.
The fabrication techniques described in the previously mentioned patent applications have been used successfully to produce multielement high performance linear arrays with totally evaporated leads for applications (e.g. FLIRs) in which high reliability and cost effective fabrication are required. For hybrid mosaic array applications, however, where close spacing of detector elements is necessary, the beveling process uses up valuable space and may induce damage in adjacent rows of detectors.
Another form of thin film interconnect is shown in U.S. Pat. No. 3,965,568 by R. W. Gooch. In the Gooch patent, a thin film lead is deposited across a planar surface to make contact to a detector. While the method of the Gooch patent appears to be useful in the formation of linear arrays of infrared detectors, it does not appear to be particularly well suited for two-dimensional detectors arrays, in which detectors are arranged in rows and columns.